The present invention relates to a ceramic insert for use with a clamp type cutting tool used mainly for high-speed cutting of cast iron and having an insert holder for clamping an insert.
Conventional cutting tool inserts for machining automotive and other machine parts include ones made of WC--Co material, called "cemented carbide", or TiN--Co material called "cermet", and ones optionally having a ceramic film formed on the substrate. Requirements for higher efficiency machining of such inserts are growing especially in recent years. Also durable cutting tools are desired which withstand high-temperature environments during machining, and allow machining at high feed and large depth of cut.
Under such machining conditions, tools are typically exposed to air heated to 1000.degree. C. or higher. At such high temperatures, cutting tools made of cermet tend to suffer a marked drop in oxidation resistance and mechanical strength. Thus, these tools are difficult to use stably. Cutting tools made of silicon nitride ceramics containing silicon nitride or SIALON as its main component, are gathering much attention these days because of their high strength and oxidation resistance at high temperatures. For example, a high-strength, high-toughness silicon nitride ceramic insert is disclosed in Japanese PCT patent publication 6-505922. This insert has a through hole formed in the center of its rake face to fasten the insert to a holder.
One problem with such silicon nitride ceramic inserts is that they tend to chip during heavy-load machining due to low toughness, though their high-temperature strength (or bending strength) is high. To prevent chipping, it is necessary to prevent the inserts from shifting during machining, and various attempts have been made to solve this problem. For example, in the above publication, a through hole formed in the rake face in the thickness direction is used to fasten the insert to the holder. But the through hole itself can be a cause of decreased strength of the insert.
To solve this problem, European Patent No. 0075177 and its corresponding Japanese patent publications 58-59705 and 58-59706 disclose an insert having a recessed portion formed in the rake face to fasten the insert by pressing its recessed portion from above with a holder presser with a clamp arm. Because the insert is made from a material that is difficult to machine, not so high dimensional accuracy is required for the recessed portion. The presser is pressed at one point against the inner wall of the recess with the tension in the longitudinal direction of the arm of the presser body to keep the insert stationary. According to these publications, the presser tip has a semicircular vertical section, and the recess formed in the insert rake face has a gourd-shaped, oval or star-shaped section for one-point contact with the presser tip. But in this arrangement, if the contact point shifts due to heavy load during high-speed machining or variations in size of presser tip, the insert may be damaged due to unstable support of the insert or stress concentration on the support point. Similar trouble may occur if minuscule vibrations are transmitted to the entire holder from an old machining equipment.
In examined Japanese utility model publication 51-24626, similar insert mounting arrangements are disclosed, in which the insert has an inverted conical recess and the presser is pressed against one side of the recess (Example 1). There is also another example in which a through hole for fastening is further formed in the bottom of the inverted conical recess (Example 2). In these examples, the insert recess inner surface may be a rough surface formed only by sintering and having a roughness value Rmax of 10 .mu.m or over. But even in these examples, it is difficult to reliably keep the insert stationary under heavy load during high-speed machining. In Example 1, since the inclined surface of the recess of is rough, it is difficult to hold the insert stably under a heavy load for a long time. In Example 2, besides this problem, the through hole tends to lower the strength of the insert. Further, if the inclination angle of the side of the conical recess is small, the component of the clamping force for stopping the shift of the insert tends to decrease, making it more difficult to keep the insert stationary especially during high-speed machining.
A main object of the present invention is to provide a silicon nitride ceramic insert which is free of the above-listed problems.